Question

Rubisco is an enzyme which acts both as carboxylase (carboxylationduring photosynthesis) and oxygenase(during photorespiration). But rubisco carries out more carboxylation in c4 plants. In c4 plants, initial fixation of carbon dioxide occurs in mesophyll cells.

Answer 1

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Answer 2

Answer:

The affinity of RUBISCO is much higher than its affinity for Oxygen. It is the concentration of Oxygen and CO2

• that determines the binding of the enzyme. Mesophyll cells of C4 plants lack this enzyme but are found in the bundle sheath cells that girdle the vascular bundles where the Calvin cycle occurs.

• Rubisco functions as oxygenase when the concentration of Oxygen is higher and it acts as carboxylase when the concentration of CO2 is high. In the mesophyll cells, the primary carbon dioxide acceptor is a three carbon compound – phosphoenol pyruvate which is converted into a four-carbon compound, oxaloacetic acid or OAA. This is converted further into malic acid which is transported to the bundle-sheath cells where it undergoes decarboxylation and carbon fixation takes place through the Calvin cycle which prevents RuBisCo to serve as an oxygenase.